The invention relates to a heater fired with liquid fuel, especially a vehicle heater with a liquid heat transfer medium, such as water, said heater comprising a tubular combustion chamber and a pot-shaped heat exchanger mounted over the combustion chamber leaving an annular space to conduct hot exhaust gas, said heat exchanger axially deflecting the exhaust gas, and bearing ribs on its inner lining which project into the annular space, said ribs being aligned essentially radially and parallel to the lengthwise axis of the combustion chamber and the heat exchanger.
In known heaters of this type (Webasto Publication 771,000), the ribs are formed of simple continuous U-shaped ribs, which define exhaust gas flow channels between their radial blades, in which channels the exhaust gas flows from the axial deflection point in the direction of an exhaust gas exhaust pipe.
A principal object of the present invention is to further improve the efficiency of the heat exchanger in a heater of this type.
This object is achieved, according to preferred embodiments of the present invention, by at least a portion of the ribs being equipped with means for exhaust gas vorticization over at least a portion of their lengths.
Heat exchangers for vehicle heaters are already known in which a plurality of individual short ribs are disposed, to increase the heat exchange area, nearly transversally to the main flow direction of the hot exhaust gases in the annular space between the combustion chamber and the heat exchanger inner lining. An arrangement of this kind, however, is not only costly from a manufacturing standpoint, but also leads to a high flow resistance to the exhaust gases and consequently a highly undesirable back pressure on combustion. The efficiency of the fan that delivers the combustion air and, consequently, the electrical power draw of the heater are increased disadvantageously.
In contrast, the present invention enables relatively low exhaust gas flow resistance to be achieved, along with an improvement of the efficiency of the heat exchanger through exhaust gas vorticization, said improvement leading to a lower exhaust temperature and, consequently, a reduced exhaust gas volume. This makes it possible to reduce the cross section of the exhaust pipe, resulting in a price reduction for the heater and facilitating its installation.
To produce exhaust gas vorticization, in accordance with preferred embodiments of the present invention, rows of essentially transverse slots are formed in blade arms of lengthwise extending U-shaped ribs, the slots being advantageously opened toward the radially inner edge of the blade arms. The exhaust gas vorticization produces a self-cleaning effect, and contamination of the spaces between the ribs is more difficult.
Greater vorticization and, consequently, a further improvement in efficiency for the heat exchanger can be achieved by displacing blade arm segments, delimited in the lengthwise direction of the ribs by successive pairs of slots, out of the blade arm plane over at least a portion of their extent. The rib segments can advantageously be displaced relative to the lengthwise axis of the ribs. For obtaining such displacing of the blade arm segments, they may be twisted about an axis which runs essentially perpendicularly to the lengthwise axes of the ribs, or may be bent out of the rib plane about an axis which is essentially parallel to the lengthwise axes of the ribs. According to another alternative, the slots can be in the form of essentially L-shaped cuts and the free parts of the rib segments can be bent outward, out of the rib plane, by twisting.
The ratio between the mutual spacing of adjacent slots, in the lengthwise direction of the ribs, and the depth of the slots is advantageously in the range from 5:1 to 0.5:1, and is preferably on the order of 1:1. At much higher ratios, the vorticization effect is indesirably weak. Excessively low values for this ratio, on the other hand, produce numbers of slots that are so great that the parts of the rib areas that are involved are lost to heat transfer.
In order to keep area losses of ribs actively involved in heat transfer small, the slot width is advantageously minimized. Thus, sawn and, therefore, relatively narrow slots are better than wide, punched slots.
According to a modified embodiment of the present invention, turbulence-creating bodies can also be inserted between circumferentially adjacent ribs as a means of exhaust gas vorticization. In particular, vorticizing strips of sheet metal can be used as turbulence-creating bodies, said strips being twisted about an axis which is parallel to the lengthwise axes of the ribs.
Preferably, the ribs are free of exhaust gas vorticization means in the vicinity of the gas reversal point, because the heat load is greatest in this area, and there is a danger than the vorticization means, especially twisted ribs, will burn away relatively rapidly at this point, resulting in gradual destruction of the heat exchanger.
Advantageously, the ribs can be formed of essentially U-shaped blades, whose central ribs are connected with the inner lining of the heat exchanger and which are distributed circumferentially at evenly spaced intervals over the inner lining of the heat exchanger.
Instead, L- or V-shaped blades can be provided for forming the ribs. It is also possible to install the ribs individually on the inner lining of the heat exchanger or to insert them through the inner lining.
These and further objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, several embodiments in accordance with the present invention.